Optical communication is a method of communication playing a central role of future broadband communications. Thus, for a widespread use of such communication, there have been demands for achieving higher performance, downsizing, and price reduction of optical components used in an optical communication system. One of potential candidates for a next-generation optical communication component that satisfies such demands is an optical communication device using a photonic crystal. This has been already partially in practical use, such as a photonic crystal fiber for polarization dispersion compensation and the like. Further, development of an optical multiplexer/demultiplexer or other kinds of devices used for wavelength division multiplexing (WDM) has been currently promoted for practical use.
A photonic crystal is made of a dielectric body provided with a periodic structure. This periodic structure is typically formed by periodically arranging, in a dielectric main body, regions (“different refractive index regions”) having a refractive index different from that of the dielectric main body. The periodic structure forms a band structure related to optical energy in the crystal, thereby forming an energy region where light propagation is impossible. Such an energy region is called “Photonic Band Gap (PBG)”.
By providing an appropriate defect in this photonic crystal, an energy level (“defect level”) is formed in the PBG, so that only light of a frequency (or wavelength) corresponding to the defect level can exist near the defect. A defect formed in a point-like shape can be used as an optical resonator for light of the frequency, and a defect linearly formed can be used as a waveguide.
As one example of the technology described above, Patent Document 1 describes a two-dimensional photonic crystal having different refractive index regions periodically arranged in a main body (slab) thereof and having a waveguide formed by linearly providing a defect in the periodic arrangement and also a resonator formed by providing a point-like defect adjacently to this waveguide. This two-dimensional photonic crystal functions as a demultiplexer that extracts, to the outside, the light of the frequency corresponding to a resonance frequency of the resonator, out of light of various frequencies propagating through the waveguide. This two-dimensional photonic crystal also functions as a multiplexer that introduces aforementioned light from the outside to the waveguide.
Many two-dimensional photonic crystals, including the one described in Patent Document 1, are designed so that the PBG is formed large for either one of TE polarized light, in which the electric field oscillates in parallel to the main body, and TM polarized light, in which the magnetic field oscillates in parallel to the main body. In either case, the PBG may not be formed for the other polarized light or, even if it is formed, optimum condition may not be necessarily provided for the aforementioned other polarized light.
For example, in a case where a photonic crystal is designed so that a PBG (TE-PBG) is formed for TE polarized light, and a defect level (or resonance frequency) is provided by a point-like defect (or resonator) in the TE-PBG, a PBG for TM polarized light (TM-PBG) may not be formed in a frequency region of this TE-PBG. In this case, TM polarized light having this resonance frequency does not resonate with this resonator. Thus, in demultiplexing, the light of this resonance frequency from among light passing through a waveguide provided near this resonator, TE polarized light can be extracted almost completely while TM polarized light cannot be extracted, thus resulting in poor demultiplexing efficiency. The same applies to multiplexing.
[Patent Document 1] Unexamined Japanese Patent Publication No. 2001-272555 ([0023] to [0027] and [0032], and FIGS. 1 and 5 to 6).